A pixel circuit in an organic light emitting diode (OLED) display implements a display function by controlling a driving current flowing through the OLED via a drive transistor. The magnitude of the driving current is related to characteristic parameters of the drive transistor, including the threshold voltage. The characteristic parameters of the drive transistor need to be compensated in order to avoid display defects due to the drift of the characteristic parameters of the drive transistor.
The compensation approaches may include internal compensation and external compensation. The internal compensation generally involves addition of new circuit elements in the pixel circuit to allow the driving current to be independent of the threshold voltage of the drive transistor. The internal compensation can be performed during the display operation of the pixel circuit. Therefore, the internal compensation can instantaneously follow a variation of the threshold voltage of the drive transistor, but cannot compensate for other characteristic parameters of the drive transistor. The external compensation generally involves the use of an external circuit to detect the characteristic parameters of the drive transistor and adjust a data voltage supplied to the pixel circuit based on the detection. The detection of the characteristic parameters requires special drive timing and is typically only performed during non-display operations of the pixel circuit, for example, when the display is powering up or down. Therefore, the external compensation cannot follow the variation of the threshold voltage instantaneously. This may affect the effect of compensation.